If I could pick a look, I would pick "unremarkable". I'm a large-ish guy, though, and I've never blended into crowds well. Just as well, as I hate crowds. My size has always been a pretty good deterrent. I'm big enough to be trouble, but I don't have the manner of "overt threat". I try to go for "too big to be easy prey, too shabby to be rich prey". Keeping alert goes a long way, definitely.
My natural expression is not friendly. Once, waiting for the school bus in 7th grade, I just glanced at a passing driver. The man stopped and got out of his truck, proceeding to yell at me for giving him a dirty look. I just looked at the man, expressionless, or so I had thought. That was the day I realized I had been gifted with a "mean face". I'm a very pale white guy, but I have squinty, Asiatic eyes (Maybe my Norse ancestors came from a bit further East, originally). Since I always look like I'm squinting my eyes, it looks like I'm glaring at everything I see. I have had to make a habit of wearing a false expression so as not to glare at people without realizing it. It's been troublesome... sometimes, I feel like I don't know who I really am anymore. I also try to moderate my expression by being polite, especially in a store or something. Last thing I need is for a cashier to think I'm giving him the evil eye.
I don't live in a great area, and I don't take my safety here for granted. I make it a point to be aware of my surroundings even when going to the corner liquor store for beer and smokes. These places get robbed a lot, so I have good reason to be wary.
Mostly, though, it's better not to be noticed at all. If you're going to stand out, either because you're really out of place, or because there's nobody around to camouflage you, then coming across as "not worth the trouble" is a pretty good substitute.